1. Field of the Invention
The present invention relates to a honeycomb structure used as a carrier in various catalysts for exhaust gases.
2. Description of the Related Art
In conjunction with the heightened awareness of environmental problems in recent years, keen interest has also been shown with respect to the clean-up of exhaust gases emitted from various boilers. For example, since large amounts of NOx and the like are contained in exhaust gases from an oil-fired boiler, a coal-fired boiler, and the like, a denitration apparatus is used to remove NOx.
As a catalyst used in this denitration apparatus, as disclosed in JP-A-2-187147, for example, discloses a catalyst in which a carrier, which is formed by causing paper fabricated of silica-alumina or alumina ceramic fiber to be impregnated with a mixture of titania sol and silica sol and by drying and baking the impregnated paper, is made to carry vanadium oxide which is a catalyst component. In addition, a catalyst is also known in which paper fabricated of glass fiber by using an inorganic binder is made to carry a catalyst component.
However, the paper which is obtained by being fabricated of the conventional ceramic fiber or glass fiber has a limit to its thickness, and has been difficult to be formed into a thickness of 1 mm or thereabouts which is required for imparting strength, for example. In conjunction with this limitation, the held amount of a hardener such as titania or silica is small. In particular, in the case of the glass fiber, since it lacks in the water holding property, its held amount of the hardener is extremely small, and since the hardener is liable to be maldistributed, it has been necessary to increase the number of processing.
Although there is still a strong demand for increasing the throughput by raising the flow rate of the exhaust gases, the paper which is obtained from the conventional ceramic fiber or glass fiber is thin as described above, and has a small amount of the hardener, so that it cannot be said that its mechanical strength is sufficient, and deformation or breakage is liable to occur. Therefore, the situation is such that it has been unable to cope with such a demand. Furthermore, with the paper made of the glass fiber, the hardener is liable to be maldistributed in the thickness direction, so that there has been a problem in that the mechanical strength differs locally.
In addition, a fairly large amount of dust is contained in the exhaust gases from the oil-fired boiler, the coal-fired boiler, or the like, and the dust is deposited on the catalyst with declining in the catalytic efficiency. To prevent this, it is considered desirable to adopt a honeycomb structure in which the aforementioned paper formed into a corrugated cross-sectional shape, as shown in FIG. 1, is laminated in a multiplicity of stages. However, the glass fiber, in particular, has poor formability due to its rigidity and brittleness, and cracks, fractures, and the like occur, so that it has been extremely difficult to form the honeycomb structure.
Moreover, since the paper which is obtained from the conventional ceramic fiber and glass fiber holds a small amount of the hardener, as described above, the resistance against wear due to dust is also low, so that it has been necessary to replace the catalyst frequently.
The invention has been devised in view of the above-described circumstances, and the object is to provide a honeycomb structure which excels in the mechanical strength and machinability, and which is suitable for a carrier of a catalyst apparatus for exhaust gases which contain a large amount of dust particularly from an oil-fired boiler, a coal-fired boiler, or the like.
To attain the above object, the invention provides a honeycomb structure characterized in that a formed piece obtained by forming paper made by mixing glass fiber and inorganic fiber. The inorganic fiber is other than the glass fiber, and the proportion of its diameter with respect to the glass fiber is 0.2 to 0.7. The formed piece is formed into a honeycomb shape and impregnated with inorganic oxide particles and an inorganic binder.